Full Case-Insensitive Language Support

With this release, Xtext supports case-insensitive languages, in
addition to case-insensitive keywords support, which was introduced
in an earlier milestone. The ConcreteSyntaxValidator is now enabled
by default during serialisation and can be registered as an EMF
validator, allowing it to integrate with existing EMF-based
applications. In the serialiser, comments are now associated with
EObjects from the semantic model, and enum literals and datatype
values are considered during backtracking.